A photochemical theory of the etiology of senile cataract is developed on the assumption that observed prevalences are representable by cumulants of tolerances, the latter being distributed normally and depending in a quantitatively defined way on the amount of short-wavelength radiation absorbed during life. The theoretical risk-curves are consistent with the observed ones. The variation of the predicted and observed values has a slope not differing significantly from unity, but the intercept differs from zero. It is concluded tentatively that the photochemical hypothesis accounts for the observations better and better as age advances, and more so where the radiation is more intense.